Modifying the contents of a message

Axel axel.grude at
Sat Jan 28 18:20:09 UTC 2012

On 27/01/12 18:29, Irving Reid wrote:
> Any modifications you make to a message in your (received) copy would cause the 
> sender's signature to no longer match, so it will be obvious that the receiver's 
> copy isn't what the sender signed.
> While I haven't looked into Outlook's signing, there's no way for it to permanently 
> alter a message and keep the existing signature. They may be doing something like 
> "person A signed the original content of the message, and person B altered the 
> message and signed the altered version" but there's no way to make it appear that 
> person A signed the altered version unless you have person A's private key.
>  - irving -

Does this also apply to header fields? I would really be interested in the edge case 
of the ISP modifying the subject lines or from Address (as certain SMTP servers would 
do). Would this also change the signature, or is this message body only?

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